1. Field of the Invention
The present invention relates to an AT-cut quartz resonator which is adapted so that a deviation of its resonance frequency from a target value, which is attributable to an error of the angle of cutting out a quartz substrate from an artificially grown quartz crystal, can be corrected through a simple modification of the electrode structure and hence kept within acceptable limits, thereby eliminating the addition of a temperature compensating circuit to the incorporation of the quartz resonator into an oscillation circuit in consumer-oriented electronic products and negating the need for adjustment of the temperature compensating circuit in industrial equipment.
2. Description of the Prior Art
Heretofore, quartz oscillators have been widely used as reference frequency generators for communications equipment, industrial equipment, consumer-electronics products, and so forth because of their excellent characteristics such as small size, high frequency accuracy and high frequency stability. Quartz resonators are also used in a wide variety of communications equipment in large quantities; for example, a plurality of quartz resonators are used to form a crystal filter, or a quartz resonator, an amplifier circuit and a temperature compensating circuit are used, in combination, to form a temperature compensated crystal oscillator (TCXO). The oscillation frequency of the crystal oscillator depends on the resonance frequency of the quartz resonator forming the oscillator. The quartz is a physically stable piezoelectric crystal, whose resonance frequency also has extremely high stability. In particular, an AT-cut quartz resonator has excellent temperature-frequency characteristics, and for this reason, it is frequently used in various fields.
The AT-cut quartz resonator has, as is well-known in the art, a pair of opposed electrode patterns for excitation deposited on both sides of a quartz substrate. The quartz substrate of an AT-cut quartz resonator is a Y plate having an angle xcex8 about the X axis of 35 degrees 15 minutes obtained by cutting out it of artificially grown quartz crystal. The resonance frequency of the AT-cut quartz resonator depends on the thickness of the quartz substrate.
This quartz resonator is placed and sealed in a package provided with a required support structure to form one piezoelectric device. Further, a quartz oscillator is formed by inserting the quartz resonator in an oscillation circuit loop constructed using chip parts or the like on a printed board. While the AT-cut quartz resonator is small in the amount of the change in the resonance frequency with a change in temperature as compared with quartz resonators of other cutting angles as referred to above, it is known in the art that the AT-cut quartz resonator shows a cubic-like temperature-frequency characteristic with a point of inflection at about 27xc2x0 C., for example, as depicted in FIG. 5.
But the temperature-frequency characteristic varies with the angle xcex8 for cutting out the quartz substrate from quartz crystal. That is, the temperature-frequency characteristic of the AT-cut quartz resonator becomes a function of the cutting angle xcex8, forming the cubic curve having an inflection point at 27xc2x0 C.
FIG. 6 is a graph showing temperature-frequency characteristics of ordinary quartz resonators using three kinds of quartz substrates of slightly different cutting angles. For example, in Japan specifications about a reference frequency source of a pager (beeper) as consumer-oriented communication equipment require that a frequency deviation in a temperature range of xe2x88x9210xc2x0 C. to 50xc2x0 C. be held xc2x12 ppm. Assume, for instance, that the curve A in FIG. 6 is representative of the frequency deviation which meets the above mentioned specs. In this instance, the specs ought to be met by a quartz resonator using a quartz substrate cut out at an angle that provides the temperature characteristic corresponding to the curve A.
However, even if an attempt is made to cut out from quartz crystal a quartz substrate which meets such specs, a wide range of variations in the cutting angle of the AT-cut quartz substrate results from limitations on the machining accuracy of a cutter for cutting the quartz substrate out. With a greatly varied cutting angle, a frequency deviation of around xc2x15 ppm occurs in the temperature range of xe2x88x9210xc2x0 C. to 50xc2x0 C. On this account, percentages of good products that meet the specs of consumer-oriented equipment are as low as 20% to 30%, constituting a major factor in raising the cost of production of quartz resonators.
To meet the specs, it is general practice in the prior art that quartz resonators of frequency deviations exceeding xc2x12 ppm in the temperature range of xe2x88x9210xc2x0 C. to 50xc2x0 C. are each incorporated into a temperature compensated oscillation circuit added with a temperature compensating circuit. However, the temperature characteristic of every AT-cut quartz resonator shows a different temperature characteristic curve; therefore, the use of such a temperature compensating circuit for each quartz resonator involves an additional step of adjusting values of individual elements of the compensating circuit in the manufacture of consumer-oriented equipment, inevitably resulting in an increase in the cost of production.
Specifications about the quartz resonator for use in industrial communication equipment such as a cellular telephone also require that the frequency deviation in a range of xe2x88x9230xc2x0 C. to 75xc2x0 C. be held within xc2x12 ppm. It is impossible, however, to realize an AT-cut quartz resonator that meets such specs, now matter what cutting angle is chosen and no matter how much the cutting accuracy is improved.
For the reasons given above, a temperature compensating circuit is essential to an oscillator for use in industrial equipment. As in the case with consumer-oriented equipment, variations in the cut angle of the quartz substrate leads to dispersion in the temperature characteristics of quartz resonators, and values of individual elements of each compensating circuit need to be adjusted. Since this adjustment operation is extremely cumbersome, the addition of such a manufacturing process raises the cost of production of industrial equipment.
Such disadvantages as mentioned above ought to be completely overcome simply by raising a yield rate of quartz resonators. That is, if quartz resonators which fulfill various specs can be offered with high productivity and with a high yield rate, it will be possible to fabricate an oscillator for consumer-electronics products which follows the specs without inserting the temperature compensating circuit in the oscillation circuit; in industrial equipment, it will also be possible to eliminate the need for adjusting the temperature compensating circuit in the oscillation circuit. This ought to improve the productivity of various communication and electronic gears and cut their manufacturing costs.
With a view to obtaining an oscillator for consumer-oriented products with no temperature compensating circuit and obviating the need for making adjustments to the temperature compensating circuit in industrial equipment, the object of present invention is to provide AT-cut quartz resonators which do not exhibit a wide range of temperature characteristic variation, that is, AT-cut quartz resonators whose temperature characteristics fall within a range of xc2x12 ppm, for example, in the temperature range of xe2x88x9210xc2x0 C. to 50xc2x0 C.
To attain the above objective, the invention of claim 1 is an AT-cut quartz resonator which has excitation electrodes formed on two principal surfaces of an AT-cut quartz substrate and which is characterized in that the above-described two electrodes are displaced a predetermined amount apart in a direction orthogonal to the X-axis direction so that a frequency deviation in a temperature range of from xe2x88x9210xc2x0 C. to 50xc2x0 C. is less than xc2x12.5 ppm.
The invention of claim 2 is an AT-cut quartz resonator which is characterized in that when the relative distance between the above-described two electrodes in the direction to the X-axis direction is represented by d, then 50 xcexcmxe2x89xa7dxe2x89xa7or 0.5 mm.
The invention of claim 3 is an AT-cut quarts resonator which is characterized in that at least one of the sides of at least one of the above-described two electrodes along the X axis is trimmed off substantially the entire length thereof.
The invention of claim 4 is an AT-cut quartz resonator which has excitation electrodes formed on two principal surfaces of an AT-cut quartz substrate and which is characterized in that when the cutting angle of the above-described quartz substrate differs from a target value, the above-described two electrodes are displaced a predetermined amount apart in a direction orthogonal to the X-axis direction so that a frequency deviation of the-above-described quartz resonator in a predetermined temperature range is smaller than a desired value.
The invention of claim 5 is an AT-cut quartz resonator which is characterized in that at least one of the sides of at least one of the above-described two electrodes along the X axis is trimmed off substantially the entire length thereof.
The invention of claim 6 is an AT-cut quartz resonator in which vertically opposed electrodes on both principal surfaces of a quartz substrate are slightly displaced apart in opposite directions along the Zxe2x80x2 axis of a quartz crystal and which is characterized in that a balancing load is formed on a piezoelectric substrate on the side opposite to the direction of displacement of the above-described electrodes, thereby slightly tilting an electric field between the above-described electrodes at their end portions and changing the apparent cutting angle due to the mass effect of the above-described balancing loads to correct the frequency-temperature characteristic of the above-described resonator.
The invention of claim 7 is an AT-cut quartz resonator which is characterized in that the above-described balancing load is formed along each of the above-described electrodes in an adjacent relation thereto.
The invention of claim 8 is an AT-cut quartz resonator which is characterized in that its frequency-temperature characteristic is slightly corrected by trimming off the above-described balancing load by a laser or electron beam.
The invention of claim 9 is an AT-cut quartz resonator which has excitation electrodes formed on two principal surfaces of an AT-cut quartz substrate and which is characterized in that adjustment regions are formed by removing surface regions of the above-described AT-cut quartz substrate, as required, along its marginal edges extending in parallel to the X axis.
The invention of claim 10 is the AT-cut quartz resonator of claim 9 which is characterized in that the above-described adjustment regions are formed along two point-symmetrical marginal edges of the above-described AT-cut quartz substrate extending in parallel to the X axis.
The invention of claim 11 is the AT-cut quartz resonator of claim 9 or 10 which is characterized in that the above-described adjustment regions are each formed along the entire length of the above-described marginal edge or along some part of the length thereof.
The invention of claim 12 is the AT-cut quartz resonator of claim 9, 10, or 11 which is characterized in that the above-described AT-cut quartz substrate is rectangular or circular in plan configuration.
The invention of claim 13 is the AT-cut quartz resonator of claim 9, 10, 11, or 12 which is characterized in that the above-described adjustment regions are also formed along marginal edges of the AT-cut quartz substrate extending along the Z axis other than those extending in parallel to the X axis sectional configuration of each of the above-described adjustment regions is stepped or bevelled, or a combination thereof.
The invention of claim 14 is the AT-cut quartz resonator of claim 9, 10, 11, 12, or 13 which is characterized in that the sectional configuration of each of the above-described adjustment regions is stepped or bevelled, or a combination thereof.